Electron multiplier system



AAAAA wvwro/v I I W REYNOLDS ATTORNEK F.. w. REYNOLDS Filed Jan. 4, 1941ELECTRONMULTIPLIER SYSTEM April 21, 1942.;

Patented Apr. 21, 1942 ELECTRON MULTIPLIER SYSTEM FrederickW. Reynolds,RidgewoomN. J., assignor to Bell Telephone Laboratoriesylncorpo- 7rated, New 'York, N.

York:

a: corporation of N ew' Application .miua 4, 1941",; Serial N... 373,1079 Claims. (01. 250-415) This inventionrelates to electron multipliersystems and more particularlyto methods and means for effectingautomatic gain control in such a system. H

An object ofthe invention is to provide an improved electron multipliersystem. I

In an example of practice illustrative of the invention a multistagephotoelectric electron multiplier is supplied with power from a highvoltage rectifier through a resistance type voltage divider. Thephotoelectric cathode is energized by light pulses modulated inaccordance with the tone values of a scanned picture. In order tocompensate for changes in the output current of the multiplier due tovoltage changes in the power supply from the rectifier, a compensatingvoltage .is applied between the photoelectric cathode and the firstsecondary electron emitter.

Such compensating voltage is obtained from the output circuit of apentode vacuum tube, the voltage of the control grid of which is variedin accordance with changes in the voltage of the power supply.

A simplified telephotographic system is made possible by thisinventionwherein a simplelens may be used to gather. light from thescanned pictureand a reasonably constant high voltage supply isused tofurnish .power. The quality of picture produced by this simplifiedsystemis equal to that produced by more elaborate systems previously known.

The invention is applicable to other electron multiplier systems thanthe example of practice described above. 1 p, A more detaileddescription of the invention now followshaving reference to' theaccompanyingdraWingLQ I H M Fig. 1 ofthe drawing illustrates theinvention embodied in a telephotographic transmitter.

Fig.2 illustrates a modification in the system of Fig. 1 to formanotherembodiment'of the invention. V

Referring now to Fig.1, the function of this transmitter is toproduce.carrier current modulated in accordance with the tone values ofsuccessively scanned] elemental areas of a picture.

This is accomplished by illuminating successive I elemental areasof thepicture by a chopped light beam varying in intensity according to thesme law at the carrier frequency Light reflected from the picture isfocussed on the photoelectric cathode of a multistage electronmultiplier 5 supplied with power from high voltage rectifierfi and'avoltage compensating vacuum tube 011- cuit'l. I

the circuit from oscillator I2 to light valve I1 1 The picture to. bescanned is mounted on a drum 8 which is rotated by motor 9 through anelectrically operated clutch I II and shaft I I. The motor9 iscontrolledby. an oscillator I2 through a speed control mechanism I3. 1

A carriage l4, outlined by dot-dash lines in Fig. 1, is arranged to bemovedparallel to the axis of the drum 8. I Carriage I4 carries a lightsource I 5, a lens I6, alight chopper I1, another lens I B', a smallmirror I9, a third lens and the electron multiplier 5. The lens I6condenses light gathered from source I5 on the aperture of light chopperI! which is 'a well-known type of string light valve. This light valveis energized bycarrier current from the oscillator I2 afteramplification in amplifier 34. The lens I8 forms an imageofthe apertureof light valve I! at a surface of the picture on drum 8, the imageforming light beam being reflected by the plane mirror I9. A part of thelight reflected from when start key is operated. An energizing circuitfor magnet 26 of clutch Ill is completed at the upper contact 21 ofstart key 25 and extends'from ground through battery 28, resistance 29,contact 21, energizing coil of magnet 26, auxiliary stop key 30 andautomatic stop switch 3| to ground. Magnet 26 locks up through contact32 being then energized by current from battery 33. V

When start key 25 and magnet 26 are operated through amplifier 34 isshortcircuited through contact 35 of start key 25 and contact 36 ofmagnet 26. The purpose of this is to send a start signal to. thereceiver in the form of an interruption of the carrier current normallybeing transmitted to the receiver. u

The clutch Ill is automatically disengaged when the scanning of apicture has been completed by reason of the stop pin 31 onthe carriageI4 engaging and opening the automatic sto'pswitch 3|. The scanningmechanism is then manually returned tothe starting position preparatoryto the scanning of the next'picture, The scanning output transformer 54.'tifier 6 at terminals 49 and 50 should be inmay be stopped at any timeby manually opening auxiliary stop key 30.

The structure of electron multiplier 5 is fully described in Patent No.2,245,624 of G. K. Teal, patented June 17, 1941, application Serial No.205,931, filed May 4, 1938. In addition to the photoelectric cathode 2|,electron multiplier 5 has ten secondary electron emitters 38', 38 38 ashield or screen electrode 39 and an anode or collector electrode 40.All of these electrodes are mounted within an evacuated glass containerwhich is provided with external contacts connected by lead-in wires toeach of the electrodes.

Power is supplied to the various, electrodes of electron multiplier 5from a voltage divider 4| having terminals 42 and 43 to which power fromhigh voltage rectifier 6 is applied through inductance coil 44,resistance 45 and potentiometer 46. Voltage divider 4| consists ofeleven sections R, each of 10,000 ohms and a section 41 of 3,000 ohms.Positive terminal49 of rectifier 6 is connected through :coil 44 to'terminal 43 and negative terminal 50 ofjrectifier 6 is connected toground and through resistance 45 and potentiometer 46 to terminal 42'ofvoltage divider 4|. Connection between the electrodes of multiplier 5mounted on carriage and voltage divider 4| which is mounted on a relayrack containing the rectifier 6, is furnished by a multiconductorflexible cable, four feet long, shielded with copper braiding.

In. accordance with applicants invention the voltage impressed betweenthe photoelectric cathode 2| and the first secondary electron emitter 38is the voltage drop through section 41 of voltage divider 4|supplemented by the voltage drop through resistance 48 which isconnected in series in the output circuit of vacuum tube 5|. The voltagedrop in resistance 48 is determined by the voltage impressed on controlgrid 52 of Vacuum tube 5| which voltage is dependent upon the currentflowing in potentiometer 46. The current flowing in potentiometer 46 issubstantially entirely dependent upon the voltage impressed betweenterminals 49 and 5010f the high voltage rectifier 6 since the current inthe various sections of the voltage divider 4| represented by electronspassing. between the electrodes in multiplier 5 is negligible compared,with the current therein due to the. voltage of the rectifier 6impressed across the resistance sections of the voltage divider 4|.

Compensating circuit 1 varies the voltage drop in resistance 48 in sucha way that the output current of multiplier 5 remains substantiallyindependent of voltage changes between rectifier terminals 49 and 50providing these voltage changes are within reasonable limits. Since theaction of the compensating circuit is'practically instantaneous, it isalso effective in reducing the effects of residual ripple in the outputcircuit of the rectifier.

The operation of the compensating circuit will now be described;Assuming a given illumination of the photoelectric cathode 2| and agiven voltage at terminals .49 and- 50 there will be a certain outputcurrent in inductance coil 53 and If the voltage of the reccreased forany reason, the voltages impressed on the several electrodes of themultiplier 5 will also be increased. Assuming for the movement that wedisregard the effect of the compensating circuit 1 operating throughresistance 48, such increase in voltage would increase the output of themultiplier. However, by taking advantage of the fact that the output ofan electron multiplier can be changed by changing the voltage impressedbetween the photoelectric cathode and the first secondary electronemitter, applicant has devised a circuit to change such voltage justenough to offset a change in multiplier output due to changes in theoutput voltage of the rectifier. Under the condition of increasedrectifier voltage described above the voltage drop across resistance 48is sufficiently reduced to keep the output of the multiplier constant.When the voltage of the rectifier increases, the current through theresistance 45 and potentiometer 45 also increases. This increases thenegative potential on control grid 52 and decreases the output currentof vacuum tube 5| flowing through resistance 48 thereby reducing thevoltage drop across resistanc 48 and the total voltage impressed betweenphotoelectric cathode 2| and secondary electron emitter 33'. Suchdecrease in voltage reduces the output of the multiplier 5 just enoughto neutralize the increased output due to the increased voltageimpressed on the other electrodes. A decreased output of multiplier 5due to a decreased output voltage' of rectifier 6 is offset by anincreased current through resistance 48 resulting in an increasedvoltage between the photoelectric cathode 2| and secondary electronemitter 38' sufiicient to maintain the output of the multiplierconstant.

The output current of the electron multiplier circuit is transmittedthrough a suitable band filter 55, equalizer 56 and amplifier 5'! to atransmission circuit 58. v

The vacuum tube 5| of compensating circuit 1 is a pentode tube,preferably of Western Electric Code No. 310 vacuum tube. V The platebattery is in two sections 59 and 60, each of 67 /2 volts. The bias forcontrol grid 52 is furnished by potentiometer 6| across. which isconnected battery 62 of 45 volts. The negative terminalof batterysection.59 is connected to cathode 63 and suppressor grid 64. Thepositive terminal of battery section 59 is connected to screen grid 65and to terminal 42 .of voltage divider 4|. Th voltages represented asbeing furnished by battery sections 59 and 60, may be obtained fromsuitably rectified alternating current, V Y

The resistance of each section R, of voltage divider 4| is 10,000 ohms;of resistance 41, 3,000 ohms; of resistance45, 10,000 ohmsyofpotentiometer 46, 25,000 ohms; of resistance 48, 38,000 ohms; and ofpotentiometer 6|, 100,000 ohms.v It

, is thus seen that for a voltage of 1,200 volts supplied by highvoltage rectifier 6 across terminals 49 and 50 the current in thesections of voltage divider 4| is about 0.01 ampere. The maximumsignaling current'produced by multiplier 5 is about'lO microampereswhich is negligible in its effect on compensating circuit 1.

- The high voltage rectifier B may be of any well-known type whichprovides a reasonably constant output voltage from a commercialalternating current source. The specific arrangement illustratedin'thedrawing is a full wave rectifier comprising two diode vacuum tubes 56and 61, thefilaments of .which are supplied with heating. current fromalternating current source 68 through transformer 69. .The source 68 isconnected to transformer 69 through switches 19. Closure ,of switches70, is indicated by the lighting of lamp 'H' which isconnected in serieswith resistance 12 across the primary winding of transformer 69. Voltageis impressed across the discharge paths oftubes' t6 and 61 bytransformer 13, the primary winding of which is energized from source 68through slow operatingrelay 14 and adjustable autotransiormer 15 Theclosure of contact 16 of relay M is indicated by. the lighting of lampll which is connected in series with resistance 18 acrossautotransformer 15. The rectified current is smoothed out by filter 79comprising input condenser 80; series inductance 8| and output condenser82. Inductance coils 83 and 84 are radiofr'equency choke coils.Resistance 85 is a very high resistance connected in series withmilliammeter 35. The current output of therectifier is indicated bymilliammeter 81 which may beshort-circuited by the closure of switch 88.e Y

By the use of applicants invention in the telephotograph systemhereinbefore described the transmitted image current forany given lightintensity changes less than 0.01 decibel for a change in rectifiervoltage up to one per cent; Such a change is not noticeable in thereproduced picture. For rectifiervoltage changes up ,to three per centthe change in image current is not greater than 0.2 decibel.

A modifledform of telephotograph system is illustrated by Fig. 2 inconjunction with Fig. l.

The elements illustrated in Fig. 2 are substituted for the elements ofFig. l which lie between the dot and double dash lines X-X and Y-'Y. Inthis modified system the light chopper I1 is re-' placed by an aperturedscreen 9|, the aperture of which is illuminated by light from source l5which light is gathered by lens l6 and condensed on the aperture. Animage of the aperture in screen 9| is formed by lens I8 at the surfaceof drum 8, the rays being reflected by the mirror H), as previouslyexplained. In this modification the successive elemental areas of thepicture are illuminatedby a steady light beam instead of l by a choppedlight beam as in the system of Fig.

1. Carrier current from oscillator l2 after amplification in amplifier34 is applied between the photoelectric cathode 2| and the firstsecondary electron emitter 38' by transformer 92, the secondary windingof which is inserted in series with conductor 90. Modulation, therefore,occurs at the first stage of multiplier 5 and the output current ofmultiplier 5 is a carrier current modulated in amplitude in accordancewith the tone values of successively scanned elemental areas of thepicture on drum 8.

In the specific system having the circuit elements described inconnection with Fig. 1, supply voltage-variations at terminals 49 and 50greater than plus or minus two or three per cent are not likely to beencountered. Since such variations can be compensated by voltage changesin resistance 48 of plus or minus 20 or volts,

. there is still room for applying a carrier current having a peak valueof volts. If the specific apparatus elements specified for the system ofFig. 1 are used in the modified system according to Fig. 2, the voltageof the carrier current impressed on transformer 92 should be so adjustedthat the carrier current voltage at the terminals of the secondarywinding of transformer 92 is approximately 35 volts.

This invention is applicable to other systems employing electronmultipliers of the type described or other types of electron multiplierswherein changes in the output current of the multipier may becompensated by suitable changes in the voltage impressed between certainof the electrodes. I

What is claimed isi 1. An electron multiplier having a plurality ofelectron emitting cathodes, an output electrode, a source ofunidirectional voltage, means connecting'said source to said cathodesand said outputelectrodeto make each cathode more negative than theadjacent one on the side toward said outputelectrode, and means tosupply a supplemental voltage across. two ofsaid cathodes to compensatefor a change in the voltage of said source.

2. An electron multiplier having a plurality of electron emittingcathodes, an output electrode, a source of unidirectional voltage, meansconnecting said source to said cathodes and said output electrode tomake each cathode more negative than the adjacent one on the side toward said output electrode, and means to apply a supplemental voltagebetween the most negative cathode and the cathode adjacent thereto tocompensate for a change in the voltage of said source.

3. An electron multiplier having a plurality of electron emittingcathodes, an output electrode, a source of unidirectional voltage, meansconnecting said source to said cathodes and said output electrode tomake each cathode more negative than the adjacent one on the side towardsaid output electrode, and means including a pentode vacuum tube tosupply a supplemental voltage between the most negative l other terminalconnected through said resistance to said primary cathode, a source ofunidirectional current having its positive terminal connected to thatterminal of said voltage divider which is connected to said outputelectrode and its negative terminal connected through a controlresistance to the other terminal of said voltage divider, and aconnection from said control resistance to said control electrode tochange the potential of said control electrode in accordance with achange of voltage of said source of unidirectional current.

5. The method of regulating the gain of an electron multiplier whichcomprises applying voltages to produce a plurality of streams of electrons including secondarily, produced electrons in series, and changingthe voltage producing one of said streams in a sense opposite to achange in the voltage producing another of said streams.

6. The method of regulating the gain of an electron multiplier whichmultiplier comprises a plurality of secondary electron emittingelectrodes providing electron multiplication in succession which methodcomprises applying voltages to said secondary electron emittingelectrodes to produce electron streams from one to the other of saidsecondary electron emitting electrodes with electron multiplication,changing the applied voltage for a plurality of said emittingelectrodes, and changing the voltage on another of said emittingelectrodes in the opposite sense.

7. The method of operating an electron multiplier of the kind in whichthe electron multiplication is increased when the over-all voltage isincreased and the multiplication is decreased when the voltage of onestage only is decreased which method comprises applying to saidmultiplier an over-all voltage which is subject to change, and adjustingthe voltage applied to one stage only to neutralize a change inmultiplication whichresults from a change in the over-all voltage.

8. An electron multiplier having a plurality of electron emittingcathodes, an output electrode, a source of unidirectional voltage, meansconnecting said source to said cathodes and said output electrode tomake each cathode more negative than the adjacent one on the side towardsaid output electrode, and means efiective- 1y independent of the outputcurrent of the multiplier to supply a supplemental voltage across two ofsaid cathodes to compensate for a change in the voltage of said source.

9. An electron discharge multiplier having a primary cathode, an outputelectrode and a plurality of secondary electron emitting electrodeslocated between said primary cathode and said output electrode, anamplifying pentode vacuum tube having a control electrode, a suppressorelectrode, a screen grid, an output electrode and a cathode, a voltagedivider having one terminal connected to the output electrode of saidmultiplier and intermediate taps connected to said secondary electronemitting electrodes respectively, an output circuit for said vacuum tubeconnected between said output electrode of said tube on the one hand andthe cathode and suppressor grid on the other hand comprising two sourcesof unidirectional current and a resistance in series therebetween, thenegative terminal of said voltage divider being connected through saidresistance to said primary cathode, a third source of unidirectionalcurrent having its positive terminal connected to that terminal of saidvoltage divider which is connected to the output terminal of saidmultiplier and its negative terminal connected through a controlresistance to the other terminal of said voltage divider, and aconnection from said control resistance to said control electrode tochange the potential of said control electrode in accordance with achange of voltage of said source of unidirectional current.

FREDERICK W. REYNOLDS.

